1. Field of the Invention
Embodiments of the present invention generally relate to the fabrication of silicon solar cells and, more particularly, to a method of converting a layer of amorphous silicon deposited on a sheet of crystalline silicon to crystalline silicon by solid phase epitaxy.
2. Description of the Related Art
Solar cells are photovoltaic (PV) devices that convert sunlight directly into electrical power. Solar cells typically have one or more p-n junctions. Each junction comprises two different regions within a semiconductor material where one side is denoted as the p-type region and the other as the n-type region. When the p-n junction of a solar cell is exposed to sunlight (consisting of energy from photons), the sunlight is directly converted to electricity through the PV effect. Solar cells generate a specific amount of electric power and are tiled into modules sized to deliver the desired amount of system power. Solar modules are joined into panels with specific frames and connectors.
Typically, the p-n junction of a solar cell is formed by diffusing an n-type dopant, such as phosphorous, into the surface of a p-type silicon sheet, wafer, or substrate. One example of performing phosphorous diffusion includes coating phosphosilicate glass (PSG) compounds onto the surface of a silicon substrate and carrying out diffusion/annealing inside a furnace. Another example of diffusing a phosphorous dopant into a silicon substrate includes bubbling nitrogen gas through liquid phosphorous oxychloride (POCl3) sources, which are injected into an enclosed quartz tube in a furnace loaded with batch-type quartz boats containing silicon substrates.
When the aforementioned processes are used to form the p-n junction of solar cells in silicon substrates, additional processing steps including etching of PSG is required. In addition, the silicon substrates, on which the diffusion occurs, are usually stacked vertically in the quartz boats for insertion into the furnace. Such handling of the substrates inevitably results in breakage of some of the silicon substrates because the substrates are relatively thin, such as 0.3 mm thick or less.
Although phosphorous diffusion of the phosphorous-doped, n-type silicon material for solar junction formation may be created by the furnace type diffusion/annealing processes discussed above, these processes require performing complex gaseous diffusion processes that require many additional pre-cleaning, post-cleaning, etching, and stripping steps. For example, a layer of PSG may remain on the surface of the substrate after formation of the n-type material. This PSG layer must be removed by wet chemical etching in diluted hydrofluoric acid solutions.
Additionally, prior art techniques use separate equipment for the phosphorous diffusion and the deposition of dielectric layers for passivation of the surface of the substrate. The use of such prior art p-n junction formation and surface passivation techniques for solar cell fabrication is expensive and can result in a defective interface between the dielectric passivation layer and the doped substrate, leading to a high surface recombination velocity for the minority charge carriers.
Moreover, using gaseous diffusion/annealing processes in a furnace, as previously described, typically results in the doping of both sides of the silicon substrate. This requires removing or otherwise isolating the doped front side of the substrate from the doped back side of the substrate in order to make the solar cell functional.
Therefore a need exists for a method for forming a p-n junction at a desired depth in a crystalline silicon substrate to provide a structure for the formation of a solar cell which eliminates many of the pre-cleaning, post-cleaning, etching, and stripping steps present in the prior art, thereby providing a more economical, efficient solar cell fabrication.